In modern industry, when it comes to production downtime, very few events are overlooked without an explanation.

“Failure analysis” on failed components, and “root cause analysis” for downtime events, are common terminology you will hear if you are involved in any kind of manufacturing environment.

ControlLogix 1756-L7x, Image by Brandon Cooper

I have been in facilities where what would have been unexplainable events in previous years were able to be investigated, and root causes determined, due to the new features found in modern control system equipment.

Examples where controller logs solved downtime mysteries:

In one instance of production downtime, upon arrival the processor was found to have no program in it.

All that could be determine by looking at the controller was there was “No Project” in the controller as that was the extent of the information scrolling across the controller’s display.

However, once the Controller’s log was reviewed, it was learned that from a certain programming station the processor was placed in program mode, and then the “download” process was initiated and then “aborted” a few seconds later.

In another event, we were investigating a controller that had power cycled several times, which obviously resulted in production downtime.

The power issue was traced back to a transformer that was feeding Non-UPS power to the control system.

It turned out that occasional power dips one would expect were sometimes falling below 100vac, which resulted in the PLC chassis power cycling, as confirmed using the controller log.

Steps to capturing the controller log of a 1756-L7X controller:

Step 1) First, you’ll need to create a line of ladder code to initiate a write to the SD card located in the Logix Processor

In the below example I use a simple “BOOL” tag in front of a “MSG” instruction to toggle it on and write the log to the SD Card:

Write Log Logic, Image by Brandon Cooper

Step 2) Now setup the “Configuration” tab of the “MSG” instruction as follows:

Write Log Config, Image by Brandon Cooper

Step 3) Next setup the “Communication” tab of the “MSG” instruction as follows:

Write Log Comms, Image by Brandon Cooper

Step 4) Once you’ve added and configure the above MSG instruction, toggle the “Write” BOOL bit on and off to execute the “MSG” instruction.

Step 5) Remove the SD card from the controller and insert it into a card reader or SD slot on your computer. Then find the log file which should be on the SD Card in a path similar to the below:

Log Path, Image by Brandon Cooper

Step 6) Open the log file (in our example it’s ControllerLog_000.txt) using a text editor like Windows Notepad, or the excellent Notepad++(if you don’t have this utility, I highly recommend it – ed.)

Log, Image by Brandon Cooper


The ability to capture and view a controller’s log is a valuable feature in the L7X (and other 5×70 and 5×80 – ed.) controllers. This log can be use in a variety of situations, especially where there’s no apparent reason for a controller event.

Having this line of code in your processor will have you ready to troubleshoot at the system level for an unexpected and otherwise unexplainable event.

Editor’s note: If you’d like to know more about the ControlLogix controller log, check out chapter three of Rockwell publication 1756-pm015.

Written by Brandon Cooper
Senior Controls Engineer and Freelance Writer

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Brandon Cooper


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